Differential signal processing is used in a number of different applications. One particular application is as part of a signal processing circuit for processing an input signal, for instance an audio signal, such as an input signal from a microphone.
FIG. 1 illustrates one example of a differential signal processing circuit, which is a continuous time sigma-delta analogue-to-digital converter (ADC) 100. A differential input signal may be received at first and second inputs, INP and INN, for first and second processing paths. Each processing path has a resistor 101 coupled to the input node so as to covert the signal voltage at the input to a signal current. A quantizer 102 comprising a voltage controlled oscillator (VCO) 103 and counter 104 is arranged in each processing path to perform analogue-to-digital conversion. The VCO 103 outputs an oscillation signal with a frequency that depends on the voltage input to the VCO and the counter 104 counts the number of oscillation cycles within a frame period to give a count value indicative of the frequency of the oscillation signal and hence the value of the voltage at the input to the VCO. The output from the quantizer 102 in the second processing path is subtracted from the output of the quantizer 102 in the first processing path to provide a digital output signal DOUT indicative of the differential input signal.
The output signal DOUT is also provided to a digital-to-analogue converter (DAC), which in this case is a current DAC (IDAC) 105 which provides feedback current signals to be combined with the signal currents in the first and second processing paths. The feedback is applied before a differential integrator 106, which in this example comprises an op-amp 107 that integrates the combined signal and feedback currents to provide the voltage input to the VCO 103.
This arrangement provides second order noise shaping and analogue-to-digital conversion and provides good performance for processing of fully differential signals such as may be received from some microphones.
However some microphones do not provide fully differential signals and may instead provide a single ended input signal. It may be desirable to use the same general signal processing circuitry for processing single ended signals, as true single ended signals or in a pseudo-differential fashion. However use of a differential signal processing architecture such as illustrated in FIG. 1 with one input left floating or held at a constant DC voltage can lead to a relatively large variation in common mode voltage which may require the use of components able to cope with a relatively large voltage swing, which may be relatively large in size and have relatively high power requirements.